1. Field of the Invention
The present invention relates to an optical element having a binary blaze grating which is suitable to be used as an optical axis adjusting element for adjusting an optical axis of a light beam in such a manner that light beams with different wavelengths are received by a common photo detector in a two-wavelength light source type optical pickup device. The present invention also relates to a mold for molding the optical element and an optical element formed by the mold. Furthermore, the present invention also relates to an optical pickup device in which the optical element is assembled as an optical axis adjusting element.
2. Description of Related Art
A two-wavelength type optical pickup device is known as an optical pickup device for recording to or reproducing from a DVD or a CD, which is provided with a laser light source with a wavelength of 650 nm for DVD and a laser light source with a wavelength of 780 nm for CD. In recent years, a monolithic type twin-laser light source, in which two laser diodes are formed on a semiconductor substrate contained in a single package, or the like, has been used to make the two-wavelength light source smaller and compact.
In the case of the twin-laser light source, the light emitting points of both laser diodes which are disposed in parallel are positioned apart with a distance of about 100 μm. Accordingly, when one of the optical axes of the laser diodes is aligned with a system optical axis, the optical axis of the laser beam from the other laser diode is shifted from the system optical axis. In this situation, the respective return light beams, which are emitted from both laser diodes and reflected by an optical recording medium, cannot be received with a common photo detector. Therefore, one of the return light beams is diffracted by using an optical axis adjusting element such as a diffraction grating or a diffraction grating referred to as a light path-composing element for enabling both return light beams to be received by the common photo detector.
A two-light source type optical pickup device provided with such an optical axis adjusting element or a light path-composing element is known.
Moreover, in an optical pickup device, a diffraction grating is used in order to divide an emitted laser beam into three beams for producing a tracking error signal of an objective lens. Such an optical axis adjusting element or a light path-composing element is also used in conventional optical pickup devices.
It is conceivable that a diffraction grating of a binary blaze shape (binary optics) is used as an optical axis adjust element of an optical pickup device. The diffraction grating has a structure, i.e., formed in such a manner that its blaze grating face is formed on the light incident surface of the optical element in a stepped shape or is formed on the light emitting surface of the optical element in a stepped shape.
An optical element provided with a binary blaze grating is generally made of an inorganic material such as glass, and the number of levels is usually act to be 2a (“a” is a positive integer), that is, 2 to the power of “a”, such as 2, 4 or 8 levels. An “x” level (“x” is a positive integer) means a binary blaze structure having step differences of the number of (x−1). The number of levels of even numbers is determined considering the work efficiency when a face of the binary blaze grating is produced in the optical element.
In general, the step shape of a grating form face is formed by an etching process. Thus, the number of times of etching required to the number of levels is two for 3 levels or 4 levels, three for 5, 6, 7 or 8 levels as described below. In the case other than the number of levels of 2a, for example, in the case of the number of levels such as 3, 5, 6, 7, 9 and 10, the number of times of etching tends to increase in comparison with that of 2a levels. Moreover, the resist film with a complicated configuration is required, which results in the reduction of work efficiency.
Number of levelsNumber of times of etching2once3twice4twice5tbree times6three times7three times8three timesFor example, when a binary blaze grating face of 5 levels shown in FIG. 6(b) is produced by etching, it is necessary to form a resist film in such a manner that the resist film entirely covers each of the step faces already formed by respective etching processes. As shown in FIG. 6(a), in the final etching process, for example, a mask pattern 106 provided with a resist film 105 is formed on the surface of the glass substrate 100 so as to cover step faces 101, 102 and 108 having already formed and a half area of the lowest face 104. Then dry etching such as ion milling is performed from the upper side and the exposed portion on the surface of the glass substrate is etched to a depth shown by the dotted line.
Furthermore, in the case of producing step differences by etching, the working time is increased but the accuracy is lowered when the one step height of the step differences is larger than 1 μm, which results in a deterioration of the stability of the optical characteristics. Accordingly, the height of the step difference in a conventional binary blaze grating is generally set to be less than 1 μm.
The wavelength of tho laser beam for a DVD in a two-wavelength type optical pickup device is 650 nm and the wavelength of the laser beam for a CD is 780 nm. In this case, when a binary blaze grating is produced so as to transmit one of the two laser beams through without diffracting and to diffract the other, the number of levels of the step difference may be required to be other than the 2a levels. Moreover, the height of the step difference of the grating may be larger than 1 μm.
However, when the binary blaze grating having the height of a step difference of 1 μm or more is produced so as to be formed with the number of levels other than 2a by etching as described above, the cost is increased due to an increase in working time, and the accuracy and stability of the optical characteristics are lowered. Accordingly, an optical element provided with a binary blaze grating having stable optical characteristics with low cost, which is suitable to be used as an optical axis adjusting element for an optical pickup device, is not produced with high accuracy, inexpensively and simply.
In addition, when the binary blaze grating face having the number of levels other than 2a levels, that is, 2 to the power of “a”, is produced by etching, the number of times of etching is increased as described above and a forming process of the resist film becomes complicated. Therefore, the required man-hour increases, and the accuracy and the quality stability are lowered. Consequently, the conventional binary blaze grating is practically limited to the number of 2a levels and the height of the step difference is limited to less than 1 μm, and thus an optical element provided with a binary blaze grating having the number of levels other than 2a cannot be produced at a low cost.
These problems occur in the case when a binary blaze grating is produced on a mold for forming a binary blaze grating, on the surface of which a binary blaze grating is produced by applying a process such as dr etching.
An optical element for diffraction used as an optical axis adjusting element or a 3-beam generating diffraction grating is designed so as to obtain the optimal diffraction efficiency with respect to the wavelength of the laser beam used. Thereby, the loss of light quantity is reduced and the utilization efficiency of the light is improved. When the diffraction grating is not designed properly, reproduction performance of an optical pickup device is decreased.
In the case of using an optical axis adjusting element in an optical pickup device provided with a twin laser light source, a variation of the diffraction efficiency affects the quantity of a reproduced signal as it is and reproducing performance is lowered.
In view of the problems described above, it is an advantage of the present invention to provide an optical element with a binary blaze grating having the number of levels other than 2a, that is, 2 to the power of “a”, which can be accurately and inexpensively formed during a short period of time. Also, it is an advantage of the present invention to produce an inexpensive optical element accurately during a short period of time.
Also, it is an advantage of the present invention to provide a mold for molding a binary blaze grating having the number of levels other than 2a, which can be accurately manufactured during a short period of time.
Also, it is an advantage of the present invention to provide an optical pickup device using an optical element with a binary blaze grating having the number of levels other than 2a, which can be accurately and inexpensively formed.
Also, it is an advantage of the present invention to provide an optical element for diffraction which is capable of obtaining the required diffraction efficiency.
Also, it is an advantage of the present invention to provide an optical pickup device using an optical element for diffraction as an optical axis adjusting element, which is capable of obtaining the required diffraction efficiency.